OBJECTIVE: To study the thyroid growth and the production and properties of transplantable thyroid tumors in the rat: changes occurring during growth, hyperplasia and involution, growth and properties of thyroid cells and thyroid tumor cells in culture. We are also studying a variety of types of cells just recently recognized to be present in the normal thyroid gland, that may be the cells of origin of some of the different histologic types of thyroid tumors observed in the past.